The Environment Protection Authority's handling of the chemical contamination at Botany has again been questioned, with revelations officials meddled in the ''independent'' review of the case.

Documents obtained under freedom of information laws reveal the EPA asked for ''technical'' changes to be made and paragraphs to be inserted into draft versions of the review report.

And the authority drew up media releases about the outcome before the final report was presented, angering residents who have led the chorus of complaint about the EPA's behaviour.

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Len Mahony said the revelations ''undermined any faith'' residents had in the watchdog.

The independent review was requested by Environment Minister Robyn Parker and the EPA's independent governing board.

It was commissioned by the EPA board, which is independent of the department, to examine the criticisms of the EPA by an expert representing the Botany community, Andrew Helps of Hg Recoveries.

But Chris Fell, who was chosen by NSW chief scientist Professor Mary O'Kane to conduct the review, told Fairfax Media he was informed the EPA was the ''client''.

As such, he followed standard practice by allowing EPA staff to look over draft reports, fact-check and discuss changes before it was presented to the EPA board.

The EPA declined to release the letter of contract and its conditions.

The review found the EPA did not knowingly misinterpret data or lack rigour in its regulation, and had fulfilled its obligations. However, it criticised many aspects of its handling of the contamination issues.

Professor Fell strongly denied any suggestion his report was ''captured'' by the EPA, and Fairfax Media is not suggesting it was.

However, the FOI documents show the EPA fought to keep the review secret. Mr Helps, who released the document, said it was another example of the ''lack of transparency'' of the EPA.

A spokesman for Ms Parker said she believed it had been an independent process.

Controversy erupted last year after metal and chemical contamination was found in the soil around Orica's Botany site. The EPA was accused of not revealing the full results and dropping the health investigation levels it was using to reassure residents, from Residential A to a lower Recreational C.

Numerous mistakes were made in the soil analysis, and Professor Fell found this led to the community being ''confused and mistrustful''.

He found the EPA initially compared some results to Residential A levels, which was ''undoubtedly meant to reassure residents'' whose properties were close to the tested land. But when further metals and chemicals were discovered on the same site, they compared them to the recreational level because the land was being used as recreational.

Professor Fell found the EPA had chosen not to tell residents the full results until further tests were done. He criticised the authority, saying there was ''substantial community perception of lack of effectiveness'' and he made a raft of recommendations for improvements.

The EPA did not respond to questions about how many recommendations have been implemented.